Issue description

As seen in this build job https://travis-ci.org/NixOS/nixpkgs/jobs/201821348 (relevant section reproduced below for archival), separate-debug-info.sh fails on clang, which is unfortunate for packages that would otherwise be portable between linux and osx.

Steps to reproduce

Include separate-debug-info.sh into a osx-enabled build, as seen here: 53985c5

Faillog

===> LD
clang-3.7: warning: argument unused during compilation: '-Wa,--compress-debug-sections'
ld: unknown option: --compress-debug-sections=zlib
clang-3.7: error: linker command failed with exit code 1 (use -v to see invocation)